Cold drawing of polygonal shapes yields a finished shape with harder surfaces, a higher yield strength, and conformed to closer tolerances than hot formed stocks. These finished shapes are suitable for machining into hex nuts and other small parts. Heretofore, in cold drawing of polygonal shapes such as hexagons, a shape or hexagon larger than the finished polygon is used as the starting material. This leads to several problems.
Typically, manufacturers of cold drawn stocks produce a number of different shapes and sizes. Without this invention they must carry an inventory of raw material bar stock of the same shapes and sizes as their finished products. This requires a much larger inventory overhead expense than is the case if it were only necessary to keep an inventory of different sizes of round stock.
Another problem commonly encountered is the result of a twisting defect in the starting material. The starting material is a coil of a hot rolled form. In the coiling process a twist is often imparted to the stock. During the cold drawing of this type of stock material, the die through which the bar stock is being drawn acts as a straightener. The twists are compressed into the area just ahead of the die. At some point the stored twisting force becomes greater than the compressive resistance of the stock as it passes through the die. The stock then flips over. That is, one corner of the polygon or hexagon shape moves laterally across an adjacent straight side and settles into an adjacent corner. Because the bar stock is continually moving through the die, the corners are distorted as they move across the straight sides.
The whole section of stock that passes through the die as the corners are flipping must then be scrapped. This flipping over also frequently results in the fracturing of the die. Of course, this does not happen if round stock is used as the starting material because it is circumferentially symmetrical.
In the prior art when trying to draw a polygonal shape from a round bar, edge defects are often encountered. Looking at a cross-section of the stock perpendicular to its long axis as it encounters a polygonally shaped die, we see that the initial contact point is on the periphery of the cross-section of the stock and the middle of a flat side of the die. The die acts on this edge of the cross-section and pushes it back in relation to the central axis of the stock. Since this is the area of greatest deformation or deepest draft, the force is considerable. The adjacent areas of the perpendicular cross-section not contacted by the die are also pulled back. This leaves a void or lack of material at the apex of the angle between the two straight sides. If the round stock is only slightly larger than the maximum cross corner radius of the finished polygon, there will be gaps where material does not fill the corners and the piece will be defective. If the diameter of the round stock is sufficiently large to prevent this defect, the force required to draw this stock through the die is prohibitive. The larger the original bar stock the greater are the stresses of deformation. The greater the stress, the more force required to accomplish the deformation. This extra force will also act to squeeze out lubricant from between the die and the bar stock and increase frictional resistance.